RIFT (a. rift; n. Rift; f. rift; and. rift), rift zone, is a large strip-like (in plan) zone of horizontal stretching of the earth's crust, expressed in its upper part in the form of one or more contiguous linear grabens and conjugated with them block structures, limited and complicated mainly by longitudinal faults such as inclined faults and expansions. The length of the rift is many hundreds or more than a thousand km, the width is usually tens of km. In the relief, rifts are usually expressed as narrow and deep elongated basins or ditches with relatively steep slopes.

Rifts during periods of their active development (rifting) are characterized by seismicity (with shallow earthquake sources) and high heat flow. During the development of rifts, thick strata or can accumulate in them, in which large oils, ores of various metals, etc. are enclosed. to the sides, and the overlying bark is a kind of vault-like bulging. Some researchers consider these processes to be the main reason for the formation of rifts, others believe that the local uplift of the upper mantle and crust only favors the emergence of a rift and predetermines its localization (or even is its consequence), while the main cause of rifting is regional (or even global?) extension. bark. Under especially strong horizontal extension, the ancient continental crust within the rift undergoes a complete rupture, and in this case, a new thin crust of the oceanic type is formed between its separated blocks due to the igneous mafic material coming from the upper mantle. This process, inherent in the rifts of the oceans, is called spreading.

According to the nature of the deep structure of the crust in rifts and the zones framing them, the main categories of rifts are distinguished - intracontinental, intercontinental, pericontinental and intraoceanic (Fig.).

Intracontinental rifts have a continental-type crust that is thinned compared to the surrounding areas. Among them, according to the peculiarities of the tectonic position, rifts of ancient platforms (epiplatform or intracratonic) of the arched volcanic type (for example, Kenyan, Ethiopian, Fig. 1) and weakly or non-volcanic fissured type (for example, Baikal, Tanganyika) (Fig. 2), as well as rifts and rift systems of mobile belts that periodically arise and then transform during their geosynclinal development and are mainly formed at the post-geosynclinal stages of their evolution (for example, the rift system of the Basins and Ranges in the Cordillera, Fig. 3). The scale of extension in intracontinental rifts is the smallest in comparison with their other categories (several km - a few tens of km). If the continental crust in the rift zone undergoes complete rupture, the intracontinental rifts turn into intercontinental ones (the rifts of the Red Sea, the Gulf of Aden, and the Gulf of California; Fig. 4).

Intra-oceanic rifts (the so-called mid-ocean ridges) have oceanic-type crust both in their axial zones (modern spreading zones) and on their flanks (Fig. 5). Such rift ridges may arise either as a result of the further development of intercontinental rifts, or within more ancient oceanic regions (for example, in pacific ocean). The scale of horizontal expansion in intraoceanic rifts is the largest (up to a few thousand km). These rifts are characterized by the presence of transverse ruptures (transform faults) crossing them, as if shifting the adjacent segments of these rift zones relative to each other in plan view. All modern intra-oceanic, intercontinental, and also a significant part of intra-continental rifts are directly interconnected on the surface of the Earth and form the rift world system.

Pericontinental rifts and rift systems, characteristic of the margins and the Indian Oceans, have a strongly thinned continental crust, which replaces the oceanic one towards the inner part of the ocean (Fig. 6). Pericontinental rift zones and systems were formed at the early stages of the evolution of secondary ocean basins. Intercontinental and intraoceanic rifts arose at least from the middle of the Mesozoic, and possibly even earlier. Intracontinental rifts within the ancient platforms were formed starting from the Proterozoic and subsequently often experienced regeneration (so-called). Rift-like linear extension zones, later subjected to compression, arose already in (greenstone belts).

Rift zones are called very extended (mantle hundreds and thousands of kilometers long) planetary scale strip-like tectonic zones, distributed within continents and oceans, in which deep (mantle) material rises, accompanied by its spread to the sides, which leads to more or less significant transverse tension in the upper floors of the earth's crust. The most important structural expression of the extension process on the Earth's surface is usually the formation of a deep and relatively narrow (from several kilometers to several tens of kilometers), often stepped graben (symmetrical or asymmetric), limited by normal faults of great depth (the rift proper or "rift valley"), or several (sometimes a whole series) of similar grabens. The bottom of the grabens is also cut by normal faults and extension cracks. The subsidence of the bottom of grabens relative to their sides, as a rule, outstrips the accumulation of sedimentary material in them, although the latter is in many cases supplemented by their filling with volcanic products, and therefore rifts usually have a distinct direct expression in the relief in the form of linear depressions. For the most part, rifts are framed on both sides, or at least on one side, by asymmetric uplifts (sloping semi-arches, one-sided horsts, and less often horsts), to some extent broken, like grabens, by longitudinal, diagonal, and transverse cracks, normal faults, and often complicated by secondary narrow grabens. In some cases, uplift also occurs inside the rift, splitting it into two branches. The ratio of the volumes of these uplifts and rift depressions reflects the ratio of uplift and extension scales in one or another rift zone. Some of them, especially oceanic, are characterized by a significant role of transverse shear displacements, in particular, along the zones of the so-called transforming faults.

Rift zones in general and, first of all, axial grabens (rifts) have increased or even very high seismicity, moreover, earthquake sources lie at depths from a few kilometers to 40-50 km, and the stress plan in the sources is characterized by the dominance of maximum subhorizontally directed extensions, approximately perpendicular to to the axis of the rift zone. Rift zones, with rare exceptions, are characterized by an increased heat flux, the value of which generally increases as one approaches their axis, often reaching 2–3, and sometimes even 4–5 heat flux units. The development of most rift zones is accompanied by manifestations of hydrothermal activity and magmatism and, in particular, by volcanic eruptions fed from subcrustal and, in some continental rift zones, possibly also from intracrustal magma chambers. However, the scale of the magmatic process, the volumes of its products, their composition, and their association with certain stages of rifting and with certain parts of the rift zone vary extremely widely. Along with rift zones, in which magmatic activity accompanied all stages of their development, and its products cover almost their entire area and reach volumes of hundreds of thousands of cubic kilometers, there are rift zones where it manifested itself locally, sporadically, or was completely absent.

The rift zones of the oceans are characterized by a contrasting strip-like bilaterally symmetrical magnetic field, which, according to the prevailing ideas, is created in the process of rifting and, as it were, imprints its individual stages. However, the magnetic field of continental rift zones largely reflects the features of the structure of their basement and has undergone only some restructuring in the process of rifting. Rift zones are usually, though not always, characterized by gravity minima in the field of Bouguer anomalies, but in the axial parts of some of them narrow maxima are distinguished, caused by the rise of mafic and ultramafic material. However, the shapes, sizes of gravity anomalies, and the nature of the factors causing disturbances can differ significantly. As a rule, rift zones are close to the state of isostatic equilibrium.

The earth's crust in modern rift zones is somewhat thinned compared to adjacent areas, and the upper part of the mantle, at least directly below the M surface, in many of them is characterized by an anomalously low velocity of propagation of longitudinal seismic waves (7.2-7.8 km / s ) and a slightly lower density and viscosity, which is apparently due to the increased thermal regime and, in some cases, the occurrence of selective melting centers in the upper mantle. These lenses or "pillows" of decompacted mantle material are probably protrusions of the roof of the asthenosphere, reaching the bottom of the earth's crust under modern rift zones. Rift zones rarely exist in isolation; as a rule, they form more or less complex combinations. Methods of "docking" neighboring rift zones and the general plan of their grouping can be very diverse, and at the same time they differ significantly in continental and oceanic zones. We call combinations of a number of closely interconnected in space approximately the same age rift zones of a similar or different type rift systems. This term can be applied to any combination of rift zones, regardless of their size, complexity and pattern, but is mainly used in relation to those combinations of them that are characterized by the presence of differently oriented rift zones, a tree-like pattern or the presence of several semi-isolated branches, not band-like, but close to the isometric general outline. In those cases when rift zones (or their systems), combined with each other, form linearly elongated structures with a length of several or even many thousand kilometers, we call them rift belts (by analogy with geosynclial and geosynclial belts comparable in length and width). orogenic belts). The term rift system is also used to refer to all interconnected rift belts of the Earth, which together form a complex winding and branching network on the surface of our planet. In the latter case, we are talking about the world rift system. The latter, with its main branches, unites most of the Earth's rift belts (and systems). Its main part crosses the oceans, and its fading ends and branches in several regions of the Earth penetrate deep into the continents. However, within the continents (and possibly in the oceans) there are also separate, isolated rift belts and even separate rift zones that are not connected with the world rift system.

1) oceanic, or intra-oceanic, in which both the axial "rift valley" and its framing have a crust close to the oceanic, which is underlain by a ledge of mantle material with anomalously reduced seismic wave propagation velocities and density compared to those typical for the upper part of the mantle;

2) intercontinental, in which the axial part of the rift has a crust close to that of intra-oceanic rift zones, its peripheral parts have somewhat thinned and reworked continental crust, and the “shoulders” have a typical continental crust. Intercontinental rift zones, like intracontinental ones, can be formed either on platforms (the Aden and Krasnomorsky rifts) or within a young folded area (the Gulf of California rift);

3) continental or intracontinental, in which both the rift and its "shoulders" have a continental-type crust, but usually somewhat thinned, especially under the rift (from 20 to 30-35 km), fragmented, anomalously heated and underlain by a lens of a somewhat decompacted mantle material.

Mutual transitions observed in nature and close structural connections of intercontinental rifts as a result of a far advanced process of development of intracontinental rifts. At least some part of the width of the intercontinental rift zones (on the order of several tens of kilometers), apparently, is due to the pull-out or strike-slip deformation of blocks of the continental crust and the protrusion of mantle-derived material between them, while in the intracontinental rifts we mainly deal with graben-like subsidence of blocks of the continental crust with an extension amplitude of the order of several kilometers and, far from always, with the filling of opening cracks with dike-like intrusions. In turn, the intercontinental rift zones are structurally closely related to the rift belts of the Indian and Pacific Oceans, in which the process of upwelling of deep material and horizontal expansion proceeds even more intensively. However, it would be imprudent to assume by analogy that all rift zones and belts of the oceans represent a further stage in the development of intercontinental rifts and, therefore, arose as a result of an even greater separation of blocks of the continental crust. For example, with regard to the East Pacific Rift Belt, it can be stated with sufficient certainty that it is younger than the Pacific Ocean and arose on the oceanic crust. The fact that the continuation of this rift belt passes almost completely into the North American continent and is superimposed on the Mesozoic Cordillera folded region obviously indicates that the driving mechanism of rifting is associated with such great depths that are no longer affected by differences between oceans and continents, but the specific manifestations of this process on the surface of the Earth differ significantly depending on whether it affects the earth's crust of the oceans, young folded regions, platforms, etc.

The rift zones and belts belonging to the three distinguished categories differ significantly in their size, morphology of structural forms, the scale of volcanism (the largest in the rift zones of the oceans), the chemistry of its products (tholeiitic basalts in rift zones, rocks very diverse in acidity and alkalinity in rift zones). zones of the continents), the magnitude of the heat flow (the highest in oceanic rift zones), the structure of the magnetic field, the plan of stresses in earthquake sources (in continental rift zones, the vector of compressive stresses is oriented subvertically, and in oceanic zones it is usually subhorizontal and subparallel to the strike of the rift zone), etc. e. Continental rift belts are characterized by such spatial combinations of adjacent rift zones as their bead-shaped, en echelon arrangement, articulated articulation, fan-shaped splitting, the junction of three zones converging at different angles, mutual parallelism, enveloping by two neighboring zones of a relatively “rigid” block separating them , which plays the role of a kind of median massif in the structure of the rift belt. On the contrary, the rift belts of the oceans are characterized by their intersection by numerous transverse or diagonal so-called transforming faults, dividing these belts into separate transverse segments (rift zones), the axes of which seem to be displaced relative to each other.

Types of rift zones on the continents. When distinguishing types among modern continental rift zones, the following main criteria should be taken into account: a) the features of the tectonic position, the structure of the base and the previous geological history of the area that became the arena of rifting, b) the nature of tectonic structures created in the process of rifting, and the patterns of their formation, c) the role, scale and features of magmatic processes accompanying rifting, and sometimes preceding it.

Based on the first criterion, rift zones and continental belts can be divided into two main groups: 1) rift belts and platform zones (epiplatform rift belts and zones), in which rifting began after a very long (200-500 million years to more ) stage of platform or close to it development; 2) rift belts and zones of young folded structures (epiorogenic rift belts and zones), where a similar process directly followed the completion of their geosynclinal development, i.e., after the orogenic stage, or even combined with phenomena characteristic of epigeosiclinal orogeny. The epiplatform rift belts are characterized by rift zones with large single axial grabens and subalkaline or alkaline nature of the products of accompanying volcanism, often with the participation of carbonatites. On the contrary, epiorogenic rift belts and zones are characterized by combinations of many narrow grabens, horsts, and one-sided blocks, and their volcanic formations belong to the calc-alkaline series.

Most of the modern continental epiplatform rift zones are confined mainly to the protrusions of the folded base of the platforms, i.e., to areas that experienced a long-term stable uplift, and much less often to areas of development of the platform cover (Levanta, North Sea, and partially Ethiopian rift zones). In most cases, rift zones are superimposed on areas of Late Proterozoic (Grenville, Baikal) folding or tectonic-magmatic regeneration, "avoiding" areas of more ancient - Archean or Early Proterozoic consolidation, which serve as the outer "frame" of these rift belts or form inside them a kind of "rigid » middle massifs (Victoria massif in the southern part of the African-Arabian belt). Much less frequently, rift zones occur on the Epipaleozoic platform base (Rhine-Rhone area of ​​the Rhenish-Libyan rift belt). In most cases, young riftogenic structures inherit the strikes of ancient folded and faulty basement structures or "adapt" to them, forming articulated, zigzag, echelon combinations. Thus, in the process of rifting, the ancient anisotropic basement splits along the weakest directions, just as a log of firewood splits according to the fibrous texture of wood. The weakened basement zones used by the Cenozoic riftogenic structures became active at times (in the Paleozoic or Mesozoic) during a long platform development and served either as zones of increased permeability for magmatic melts and intrusions, in particular ring-type alkaline massifs, or zones of faults and grabens.

Among the epiplatform rift zones, two types are clearly distinguished, which differ significantly in the nature of structures, the relative role of volcanism, and the history of formation. The author called them fissured and dome-volcanic (Milanovsky, 1970):

a) rift zones of the dome-volcanic type (Ethiopian and Kenyan zones of East Africa) are characterized by exceptionally powerful and prolonged terrestrial volcanic activity. It begins in a wide area even before the rift is laid, and subsequently continues within the axial graben and associated secondary grabens and fault zones. The main role is played by eruptions of basic and intermediate lavas and pyroclastolites of strongly alkaline and weakly alkaline series. Acid (high alkalinity) volcanics also play a significant role in the Ethiopian rift zone. The emergence of a rift is preceded by a long growth of a vast gently sloping oval arched uplift, accompanied by powerful eruptions, then a relatively shallow graben is laid in its axial weakened zone, as well as additional grabens and normal faults associated with it - transverse and diagonal on the limbs of the arch and fan-shaped diverging on its periclines. The amplitude of horizontal extension in dome-volcanic rift zones is minimal. They are characterized by moderate seismicity. The formation of the dome, characterized by a large gravitational minimum, is apparently associated with the appearance of a lens of decompressed, anomalously heated material and with individual magma chambers in the upper mantle, and the formation of grabens is partly due to subsidence of crustal blocks during the unloading of these chambers during eruptions;

b) slot-type rift zones are characterized by a greater depth of grabens, which can reach 3–4 km (Upper Rhine graben) and even 5–7 km (South Baikal graben). Large gravity minima are associated with the large thickness of loose sediments in the grabens. Often the grabens substitute each other on the link. Marginal uplifts are much narrower than in domed volcanic rifts, are not observed everywhere, often only on one side of the graben, and sometimes are completely absent, and in some cases (the rift zone of the North Sea) rift development occurs against the background of general subsidence. In some places inside the rift zone arch- and horst-like uplifts arise, reaching in some cases a huge height (up to 4-5 km in the Rwenzori block in the Tanganyika zone). Gravity maxima are associated with internal uplifts, and their protrusion is of an antiisostatic nature. Slit rift zones are characterized by relatively weak, local and episodic manifestations of volcanism or their complete absence. On this basis, weakly volcanic (Tanganyika, Upper Rhine) and non-volcanic zones (the middle segment of the Baikal rift belt) can be distinguished among them. Eruption centers are confined to saddles between clearly arranged grabens, their marginal steps, marginal uplifts, and other uplifted areas. Petrochemically, volcanism is close to arch-volcanic zones, but extremely alkaline series (sodium or potassium) and carbonatites are more often present here. Volcanic activity can manifest itself at different stages of rifting.

The process of formation of slotted zones begins with the emplacement of narrow linearly elongated grabens (usually confined to ancient weakened zones), filled with initially fine-clastic ("molassoid"), as well as carbonate and chemogenic sediments, which are subsequently replaced by coarser clastic continental molasses. This formation series, as well as geomorphological data, show that the intensive growth of marginal and internal uplifts began later than the occurrence of grabens, and in some places has not yet manifested itself. The concept of the emergence of a rift as a result of the collapse of the dome is not applicable to slotted rift zones. These zones are more seismic than dome-volcanic ones. The amplitude of horizontal stretching in them may be greater than in the latter, but, apparently, it usually does not exceed 5–10 km. In the grabens of slotted rift zones, a significant "leakage" of thermal energy apparently occurs. In some crevice zones, in addition to the sliding one, there is a shear component. In the Levantino zone, the latter, apparently, significantly exceeds the transverse tension, and in some of its sections, the horizontal deformation approaches pure shear.

In rift belts and zones of young folded structures, rifting follows the geosynclinal cycle of development, being a direct continuation of its final orogenic stage. In the process of rifting in these zones, a system of narrow, but very extended (up to many hundreds of kilometers) mutually parallel grabens, separated by narrow horsts or one-sided horsts commensurate with them (the Cordillera rift system), often arises in these zones. The amplitudes of the relative movement of the blocks along the normal inclined faults separating them reach 2-5 km. Along with the general significant horizontal tension, significant shear deformations can occur (for example, the San Andreas shear in California). The formation of rift structures is preceded and accompanied by extremely powerful eruptions of calc-alkaline magma, both acidic and basic. Volcanoes were fed from chambers of different depths, located both in the upper mantle (foci of basaltic volcanism) and in the crust (foci of liparite-dacitic volcanism). The dispersal of extension and associated volcanism within a very wide band with numerous grabens in some epiorogenic rift zones is apparently due to the fact that rifting develops under conditions of a more “warmed up” and “plastic”, and in the upper part - fragmented lithosphere compared to relatively "hard" and "cold" lithosphere of epiplatform rift zones.

Most of today's rift zones are interconnected, forming a global system stretching across continents and oceans (Figure 5.1). Awareness of the unity of this system, which covered the entire globe, prompted researchers to look for planetary-scale mechanisms of tectogenesis and contributed to the birth of a “new global tectonics”, as the concept of lithospheric plate tectonics was called in the late 60s.

In the system of the Earth's rift zones, most of it (about 60 thousand km) is located in the oceans, where it is expressed by mid-ocean ridges (see Fig. 5.1), their list is given in Ch. 10. These ridges continue one another, and in several places are interconnected by “triple junctions”: at the junctions of the West Chilean and Galapagos ridges with the East Pacific, in the south of the Atlantic Ocean and in the central part of the Indian. Crossing the boundary with passive continental margins, oceanic rifts continue to be continental. Such a transition is traced south of the triple junction of the Aden and Red Sea oceanic rifts with the Afar Valley rift: along it, from north to south, the oceanic crust wedges out and the continental East African zone begins. In the Arctic Basin, the oceanic Gakkel Ridge continues with continental rifts on the shelf of the Laptev Sea, and then with a complex neotectonic zone, including the Momsky rift (see Fig. 5.3).

Where mid-ocean ridges approach an active continental margin, they can be absorbed into a subduction zone. So, at the Andean margin, the Galapagos and West Chilean ridges end. Other relationships are demonstrated by the East Pacific Rise, above which the Rio Grande continental rift formed on the overthrust of the North American Plate. Similarly, the oceanic structures of the Gulf of California (apparently an offshoot of the main rift zone) are continued by the continental system of Basins and Ranges.

The dying off of rift zones along strike has the character of gradual attenuation or is confined to a transform fault, as, for example, at the end of the Juan de Fuca and American-Antarctic ridges. For the Red Sea rift, the Levantian shift serves as the end.

Covering almost the entire planet, the system of Cenozoic rift zones exhibits geometric regularity and is oriented in a certain way relative to the axis of rotation of the geoid (Fig. 5.2). Rift zones form an almost complete ring around the South Pole at latitudes of 40-60° and depart from this ring meridionally with an interval of about 90° by three belts fading to the north: East Pacific, Atlantic and Indian Ocean. As shown by E.E. Milanovsky and A.M. Nikishin (1988), perhaps, with some conventionality, the fourth, Western Pacific belt, which can be traced as a set of back-arc manifestations of rifting, is marked in the corresponding place. The normal development of the rift belt here was suppressed by intense western displacement and subduction of the Pacific Plate.

Under all four belts, to depths of a few hundred kilometers, tomography reveals negative velocity anomalies and increased attenuation of seismic waves, which is explained by the upward current of the heated mantle substance (see Fig. 2.1). The regularity in the placement of rift zones is combined with global asymmetry both between the polar regions and relative to the Pacific hemisphere.

The orientation of the stretch vectors in the rift zones is also regular; The latter are maximum in the equatorial regions, decreasing along the ridges both in the northern and southern directions.

Outside the global system, there are only a few of the large rifts. This is the system Western Europe(including the Rhine graben), as well as the Baikal (Fig. 5.3) and Fengwei (Shanxi) systems, confined to northeast-trending faults, the activity of which is believed to be supported by the collision of the continental plates of Eurasia and Hindustan.

continental rifting

Active rift zones of the continents are characterized by dissected topography, seismicity, and volcanism, which are clearly controlled by large faults, mainly normal faults. The main modern belt of continental rifting, stretching almost meridionally for more than 3 thousand km across the whole of East Africa, was called the belt of the Great African Rifts. The zones that form it branch out and converge, obeying a complex structural pattern. In the rifts of this belt, lakes Tanganyika, Nyasa (Malawi) and others were formed; among the volcanoes confined to it is such a giant as Kilimanjaro, and Nyiragongo, known for its activity. The Baikal rift system is also one of the most representative and well-studied.

Relief, structure and sedimentary formations. The central position in the rift zone is usually occupied by a valley up to 40–50 km wide, bounded by faults, often forming stepped systems. Such a valley sometimes stretches along the arched uplift of the earth's crust (for example, the Kenya Rift), but it can also form without it. Tectonic blocks on the rim of the rift are uplifted to 3000-3500 m, and the Rwenzori mountain range in the north of the Tanganyika zone rises to 5000 m. Rifts are often complicated by longitudinal or diagonal horsts. In the area of ​​Basins and Ranges North America stretching of the earth's crust was distributed over a vast (almost 1000 km) area, where numerous relatively small grabens were formed, separated by horsts, which creates a complex tectonic relief. Sometimes, as, for example, in the east of the Brazilian Shield, systems of asymmetric one-sided grabens are observed. On the whole, the asymmetry of structure and topography is characteristic of many continental rift zones.

In their upper exposed part, the faults are inclined to the horizon at an angle of up to 60 degrees. However, judging by the seismic profiles, many of them flatten out at depth, they are called listric (Greek bucket-shaped). When shifting along normal faults, a strike-slip component is also often noticeable (left-sided in Baikal). For seismically active faults, fault extension and shears are also determined when solving focal mechanisms. As shown by V.G. Kazmin (1987), diagonally oriented strike-slip faults and their echeloned systems in some cases transfer movement from one opening rift to another and in this respect are similar to transform faults of oceanic rifting. In complexly built rift zones, such as the East African one, faults and strike-slips form regular and very expressive parageneses.

Dynamothermal metamorphism develops along some relatively gently sloping ruptures parallel to their displacer, which can be judged in those cases where, with further extension, the metamorphites were exposed or approached the surface.

Sedimentary formations of continental rifts, predominantly molasses, are characterized by a combination with one or another amount of volcanic rocks, up to cases when sedimentary formations are completely replaced by volcanic ones. According to E. E. Milanovsky, the thickness of the Cenozoic filling of rifts can reach 5-7 thousand m (for example, in South Baikal), but usually does not exceed 3-4 thousand m. , alluvial, proluvial, and in the Baikal depressions also of fluvioglacial and glacial origin. As a rule, the coarseness of clastic material increases from bottom to top. Under the climatic conditions of the Afar rift, the accumulation of evaporites was possible. In the zone of volcanism, the removal of matter by hydrothermal solutions also creates conditions for the deposition of specific chemogenic sediments - carbonate (including soda), siliceous (diatomaceous, opal), sulfate, and chloride.

Magmatism and its products. Continental rifting is accompanied by magmatism, and only locally its surface manifestations may be absent. So, in particular, there is no reliably established volcanism in the rift of Lake Baikal, but in the same system in the Tunkinsky and Charsky rifts there are fissure outpourings of basalts. Volcanoes are often located asymmetrically - on one side of the rift valley, on its higher side.

Igneous rocks are exceptionally diverse, among them alkaline varieties are widely represented. Contrasting (bimodal) formations are characteristic, in the formation of which both mantle basalt melts (and their derivatives) and anatectic, predominantly acidic melts, formed in the continental crust, participate. In contrasting formations of the East African Belt, along with alkaline olivine basalts, trachytes, and phonolites, V. I. Gerasimovskii and A. I. Polyakov indicate rhyolites, comendites, and pantellerites. The potassium series contains leucitites and leucite basanites. There are alkaline ultramafic rocks and accompanying carbonatites.

According to M. Wilson (1989), data on the contents of rare elements and isotope ratios of neodymium and strontium in different volcanic formations of the East African belt indicate the unequal degree of contamination of mantle magmas by crustal matter. It turned out that in some series the entire variety of rocks was due to fractional crystallization.

Geophysical characteristics. According to geophysical data, the thickness of the crust under the continental rifts decreases and a corresponding uplift of the Mohorovichic surface takes place, which is there in mirror correspondence with the terrestrial relief. The thickness of the crust under the Baikal rift decreases to 30-35 km, under the Rhine - to 22-25 km, under the Kenya - up to 20 km, and to the north, along the Afar valley, it reaches 13 km, and then under the axial part of the valley an oceanic bark.

In the mantle ledge under the rift, the rocks are decompressed (p-wave velocities vary in the range of 7.2-7.8 km/s), their elastic characteristics are reduced to values ​​typical of the mantle asthenosphere. Therefore, they are considered either as an asthenospheric diapir (for the Rio Grande and Kenyan rifts) or as a lenticular “cushion” extended along the rift zone and, to some extent, isolated from the main asthenospheric layer. Such a lens with a thickness of 17 km was discovered by seismic sounding under Baikal. It has been noted that in asymmetric rifts, the crest of the mantle protrusion most often does not coincide with the axis of the valley, but is shifted towards the higher flank. There are also centers of volcanism.

The shallow occurrence of the asthenosphere limits the depth of seismic sources. They are located in the thinned crust, and depending on its thickness, the maximum depth of the foci varies from 15 to 35-40 km. The solution of the focal focal mechanism establishes fault and shear displacements subordinate to them.

The proximity of the heated asthenosphere, volcanism and increased permeability of the crust disturbed by faults are expressed in the geothermal field, the heat flow in the rifts is sharply increased. Magnetotelluric sounding determined the high electrical conductivity of rocks in the asthenospheric ledge.

In the gravitational field, the rift zone corresponds to the negative Bouguer anomaly, which extends as a wide strip and is believed to be due to the deconsolidation of mantle rocks. Against the background, sharper negative anomalies are traced over rift basins with their loose sedimentary filling and positive anomalies marking the intrusion bands of basic and ultrabasic igneous rocks.

Mechanisms of rifting. Physical models of rift formation take into account the observed concentration of extensions in a relatively narrow band, where a corresponding decrease in the thickness of the continental crust occurs. Along the weakened zone, an increasingly thin "neck" (English, necking) is formed, up to the rupture and expansion of the continental crust with their filling with oceanic-type crust. In different rifts such critical moment apparently occurs at different limiting thicknesses of the sialic crust (in the Red Sea and Aden rifts, it was thinned by about half) and signifies the transition from continental to oceanic rifting.

Rice. 5.4. Models of continental rifting. According to R. Almendinger et al., (1987):
a - classical model of symmetrical horsts and grabens; b - Smith et al. model with a subhorizontal disruption between the brittle and ductile strains; c - model of W. Hamilton and others with lenticular character of deformations; d - B. Wernicke's model, which provides for asymmetric deformation based on a gentle fault

Since earth's surface stretching in continental rifts occurs through fault displacements, the original, classical model of rifting took into account only these brittle deformations (Fig. 5.4., a). According to the calculations of J. Angelier and B. Coletta, the total effect of displacement along faults gives an extension of 10-50% in the Gulf of Suez to 50-100% in the Californian system and up to 200% in the south of the Basin and Range region. On one of the segments of the Afar Valley, the calculations of W. Morton and R. Blakk gave a threefold extension. Such high values ​​were satisfactorily explained in later models, which were built taking into account changes in the mechanical properties of rocks with depth as pressures and temperatures increase. R. Smith's model (Fig. 5.4, b) provides for the existence of a layer of plastic deformations in the lower crust, under the layer of brittle deformations. At the same time, the normal faults bend and flatten in their lower part as they stretch, becoming listric. The subsidence of blocks along such faults is accompanied by their rotation (overturning), and the degree of extension increases from the edges of the rift zone to its center. The same effect can also be obtained under the assumption that in the middle part of the crust there is another, transitional, tier of deformations, where the displacement is dispersed over many small diagonal cleavages or subhorizontal slip surfaces.

All these variants of rifting involve local thinning of the crust under the action of tensile stresses with the formation of a symmetrically constructed rift zone. D. Mackenzie (1978) gave a quantitative assessment of the consequences of such thinning: isostatic subsidence of the crust and counter uplift of the asthenospheric ledge, to which this researcher assigns a passive role.

Another model that takes into account new data on the deep structure of continental rifts and the asymmetry inherent in many of them was proposed by B. Wernicke (1981). The leading role is given to a large gentle (10-20°) normal fault, in the formation of which, possibly, intracrustal asthenospheric layers are used (Fig. 5.4, d). In the course of extension, the hanging limb becomes complicated by a stepped system of small listric normal faults, while the other limb is dominated by a ledge corresponding to the plane of the main normal fault. The dynamothermal metamorphism mentioned above and the outcropping of metamorphites to the surface during further sliding of the hanging wing down the fault plane are associated with it. B. Wernicke's model successfully explains a number of other features of the structure and development of asymmetric rifts. When the crust is thinned by displacement along a gentle fault, the asthenospheric protrusion should be located not under the axial part of the rift, but under the hanging wall, supporting and lifting it, which is observed on many profiles. Volcanism is localized on the same high side of the rift. Such an asymmetry is well expressed in the East African belt, along which rifts alternate with a relatively elevated western and eastern limb.

Taking into account the new geophysical data, the diversity of the deep structure of the zones of continental rifting is beyond doubt. Therefore, none of the listed models can claim to be universal, and the mechanism of rift formation varies depending on such conditions as thickness, structure, temperature regime of the crust, and extension rate.

Hydraulic wedging mechanism. All of these models are based on the compensation of crustal stretching by its mechanical deformation (brittle or ductile), a decrease in thickness, and the formation of a “neck”. In this case, magmatism is assigned a passive role. Meanwhile, in the presence of basaltic magma (with its high liquid properties) at depth, a fundamentally different mechanism comes into play.

There is every reason to believe that the rapid rise of basaltic magma to the surface is provided in extension zones: the wedging effect that magma has on the rocks of the lithosphere. Ideas about this process are based on the study of linear dikes and their systems (which are considered as solidified magmatic wedges) and on the application of the theory of hydraulic fracturing of rocks to them. It was based on detailed studies of Tertiary and Paleozoic dikes in Scotland, which ended with generalizations by J. Ritchie and E. Anderson. Already decided on this material characteristics linear dikes. As a rule, they are intruded along vertical fractures by spreading the wings perpendicular to the fracture without significant compaction or collapse of the rocks hosting the dike. There is usually no fault or shear displacement during intrusion. The dikes form a subparallel system, within which the thickness of the dikes remains uniform.

E. Anderson showed the active role of magma in the formation of dikes. Intruding along a crack perpendicular to the minimum compressive stress, the magmatic melt has a wedging effect, increasing the crack in length (see Fig. 5.5, III). Further study of the dependence of the intrusive process on the ratio of principal stresses near the magma chamber was given by J. Robson and C. Barr. However, the quantitative substantiation of the dike emplacement mechanism became possible later, in connection with the development of the theory of hydraulic fracturing of rocks during oil production. M. Hubbert and D. Willis drew an analogy between artificial hydraulic fracturing and the intrusion of magmatic dikes into the earth's crust. With regard to the latter, the question was specially considered by A.A. Peck and V.S. Popov.

Hydraulic fracturing (hydrofracturing) is the process of formation and propagation of cracks in rocks under the pressure of a liquid, including magmatic melt. The stretching of the earth's crust can be expressed as gaping cracks only at the smallest depths - up to 2-3 km. Deeper, with an increase in all-round pressure and temperatures, brittle separation is replaced, as already noted, by shearing along more and more numerous planes, and then passes into plastic deformation. Since systems of basalt dikes originate at great depths, their formation by passive filling of gaping cracks is excluded. The only possible mechanism is active penetration through hydraulic fracturing followed by expansion of the fracture walls.

For hydraulic fracturing to develop, it is sufficient that the fluid pressure only slightly exceeds the minimum compressive stress in the rock; usually in calculations, their ratio is taken equal to 1.2. A hydraulic wedge is formed, the fluid front comes close to the end of the crack, but never reaches it. The wedging effect is ensured by the stress concentration at the crack tip, where the pressure expanding it increases from the tip in proportion to the cube of the crack opening in accordance with the decrease in hydraulic resistance (see Fig. 5.5, IV). The development of hydraulic fracturing is little affected by real differences in the strength of host rocks. A brittle fracture and a magmatic wedge propelling it rapidly propagate. As the calculations of N.S. Severina, the heat transfer of such an injection is compensated by the release of heat due to friction on the contacts; therefore, there is no significant increase in viscosity, which would slow down the injection process. According to seismological observations by V.M. Gorelchik and others during the fissure eruption of Tolbachik in Kamchatka, the basalt wedge rose there at a speed of 100-150 m/h.

The intrusion of a vertical dike becomes possible when one of the main compressive stresses directed horizontally is reduced by tectonic extension. Parallel dikes belonging to the same swarm, apparently, intruded sequentially: each successive hydraulic wedge created a halo of compressive stresses, which prevented other injections, and was subsequently gradually removed by tectonic extension.

Thus, in the presence of liquid magma at the depth of the reservoir, conditions arise for the growth of lithospheric layers under the action of many parallel hydraulic fractures, in each of which the injection of the melt leads to the expansion of the host rocks. The magmatic litter of the lithospheric layer injected by dikes provides the necessary freedom of horizontal sliding. Possibly alternate or joint (on different levels) manifestation of both hydraulic wedging and mechanical tension in one rift zone.

For continental rifts, the mechanism of hydraulic wedging becomes significant at the final stage of their development, when the thinning of the crust approaches critical values, and a decrease in the load on the asthenospheric ridge promotes a greater separation of basalt melts. It is under such conditions that longitudinal swarms of parallel dikes, discovered by P. Mor (1983) and associated with basalt volcanism, appear on the western side of the Afar Rift. In the Krasnomorsky rift, a similar phase began about 50 Ma and intensified 30 Ma ago, when powerful swarms of parallel dikes of contrasting composition (from tholeiitic basalts to granophyres) were intruded into the ancient granitic crust, which are traced along the northeastern coast. Only 5 million years ago, magmatic wedges concentrated in a narrow band, causing the separation of the Arabian Plate. Continental rifting was replaced by oceanic rifting, which continues to the present.

In cases where the development of a continental rift stops at an earlier stage, it persists as a weakened zone, a furrow on the continental plate, as exemplified by the aulacogenes (see Chap. 13).

5.3. Oceanic rifting (spreading)

Oceanic rifting, which is based on separation through magmatic wedging, can thus develop as a direct continuation of the continental one. At the same time, many modern rift zones of the Pacific and Indian Oceans were originally formed on the oceanic lithosphere in connection with rearrangements in the movement of plates and the death of earlier rift zones.

The assumption about the formation of the earth's crust in the mid-ocean ridges during their expansion by mantle convection, the rise and crystallization of basaltic magma was expressed by A. Holmes back in the 30s and 40s, likening the oceanic crust diverging from the active zone to endless conveyor belts. This idea has received further development after G. Hess (1960) put it at the basis of ideas about the evolution of the oceans. R. Dietz (1961) introduced the term seabed spreading(English, spread - deploy, spread). Soon G. Bodvarson and J. Walker. (1964) proposed a mechanism for the spreading of oceanic crust through dikes, which was the focus of attention at the symposium "Iceland and mid-ocean ridges" and initiated the deciphering of tectonomagmatic processes that form the crust in the spreading zone. Intensive studies of subsequent decades, including deep-sea drilling and detailed survey of spreading zones using manned underwater vehicles, provided a lot of new material for this.

Spreading in Iceland. For understanding oceanic rifting, of particular interest are data from Iceland, where the Mid-Atlantic Ridge is elevated above sea level for 350 km. The history of repeated fissure eruptions of basalts has been known there for a millennium, and since the last century, special geological studies have been carried out, which were later supplemented by geophysical and high-precision geodetic observations. Modern tectonic and volcanic activity is concentrated in submeridional neovolcanic zones that cross the island in its central part. The youngest basalts corresponding to the Brunhes epoch are confined to their axis. They are bordered by basalts with an age of 0.7-4 million years, then a thick series of plateau basalts emerges from under them up to the Middle Miocene (16 million years), occurring with a predominance of a counter slope towards neovolcanic zones. It is characteristic that in the opposite direction (from the axial zones) the basalt covers decrease in thickness and successively wedge out, starting from relatively young ones. As a result of any point II, the slope of the basalts increases from top to bottom: from a horizontal occurrence near the already eroded top of the plateau basalts to 3-4° at elevations of about 1000 m, 7-8° at sea level and approximately 20° at a depth (2000 m (according to drilling data Each fissure outpouring leaves a horizontally occurring (and wedging out across the strike of the zone) basalt cover up to 10 m thick or more, as well as its supply channel - a vertical dolerite dike, most often 10 m wide, oriented perpendicular to the axis of minimum compressive stresses, i.e. along the rift zone.Each successive eruption adds one basalt cover and one dike, so the dikes get thicker down the plateau-basalt section.This issue was specifically investigated by J. Walker in East Iceland.He found a regular decrease in the number of dikes as they rise from sea level to watershed marks 1000-1100 m and extrapolated their further decrease along a linear relationship.All such plots showed the complete wedging out of dikes at elevations of 1350-1650 m, i.e., exactly where the primary top of the plateau basalts should have been. It is assumed that below sea level the number of dikes increases accordingly.

As the plateau basalts are layered, their gravitational subsidence occurs, which is largely compensatory in relation to the feeding magma chamber, which was traced by magnetotelluric sounding. Simultaneously, as more and more parallel dolerite dikes are emplaced, they are pulled apart by the amount of their total thickness. Based on such observations, G. Bodvarson and J. Walker proposed a mechanism for the growth of the earth's crust through the intrusion of dikes. On fig. 5.5.1 from a later publication by G. Palmason (1973) this mechanism is explained by a kinematic diagram. It shows the calculated trajectories and isochrones of the movement of rocks newly formed in the axial zone during their subsequent subsidence and retraction on one side of the axis. The scheme of I. Gibson and A. Gibbs (Fig. 5.5, II) illustrates the ever-increasing inclination of plateau basalts at depth and the structure of fan-shaped monoclines that form on both sides of the axial zone as the erupting basalts subside and the core is wedged by dikes. The latter are vertical during intrusion, and later they incline together with the enclosing plateau basalts. The end result is a new formation of the second layer of the oceanic crust.

Rice. 5.5. Model of formation of the second layer of oceanic crust in Iceland, Mid-Atlantic spreading zone:
I - kinematic diagram of G. Palmason (1973): trajectories of movement of erupted basalts (dashed line) and isochrones of their movement (solid lines) in the process of expansion and isostatic subsidence. II - scheme of I. Gibson and A. Gibbs (1987), explaining the mechanism of spreading through the introduction of dikes and surface effusions of basalt: the wedging effect of the dikes determines the separation, subsidence under the load of basalts forms fan-shaped monoclines on both sides of the axial zone (K - a complex of parallel dikes ). III - intrusion of a basalt dike in a plane perpendicular to the minimum compressive stress, according to E. Anderson and M. Habert. IV - basalt dike as a hydraulic wedge: a diagram of stresses bursting open a crack (P), which sharply decrease towards the top of the hydraulic wedge in inverse proportion to the cube of the crack opening, which creates a stress concentration there, a wedging effect and wedge advancement (according to A.A. Pek, 1968) : l - crack length; d - crack opening: R to - the pressure of the injected fluid at the beginning of the crack; R b - lateral stresses compressing the crack

The real implementation of this model in Iceland is complicated by multiple lateral "jumps" of the axis of fissure eruptions within the volcanic zone and even by the displacement of this entire zone. In addition, some part of the extension is due to normal faults and open cracks, i.e., pull apart. It is believed that such structures compensate at the top for the intrusion of those dikes that did not reach the surface. In particular, shielded dikes probably terminate in dolerite sills, which are numerous among plateau basalts. In addition, during fissure eruptions, part of the basaltic magma propagates from the volcanically active area along the strike of the zone by means of longitudinal germination of dikes. According to G. Sigurdson, several such intrusions occurred after the 1975 Krabla fissure eruption, their advance at a speed of several hundred meters per hour was accompanied by seismic shocks and subsidence of the surface in a strip a few kilometers wide. The total subsidence reached 1.5 m, including the amplitude of displacement along some faults - up to 1 m.

The use of observations from Iceland, despite their detail and reliability, is limited by the anomalous nature of this segment of the mid-ocean ridge relative to normal submarine spreading zones. The thickness of the oceanic crust here is much higher than normal (up to 40 km), which steadily maintains the surface of the island above sea level throughout its geological history. Taking into account the characteristic geochemical features of Icelandic basalts, this is explained by the spreading axis passing over the mantle jet, which uplifts matter from deep parts of the mantle and increases the rate of supply of basalt melt, which forms thicker oceanic crust (see Chapters 6 and 7).

Spreading in submarine mid-ocean ridges. A number of segments of the ocean rift zones have been studied in detail by now with the help of manned submersibles. The beginning of this work was laid by the Franco-American program FAMOUS, according to which in 1974-1975. areas of the Mid-Atlantic Ridge southwest of the Azores were mapped, located in the rift valley, on the transform fault and at their junction. The seismically and volcanically active axial part of the rift valley in the studied segment turned out to be built symmetrically (see Fig. 10.1, II). On both sides of the recently erupted pillow lavas, which form embankments elongated along longitudinal fissures, products of earlier fissure eruptions were traced for a distance of 1.5 km to one side and the other, which was established from the thickness of weathering crusts on lava pillows.

Subsequently, to the south, in the area of ​​the Kane fault, similar studies under the MARK program covered several fault-separated segments of the Mid-Atlantic Ridge with a total length of about 80 km (see Fig. 10.1, I, IV, V, VII). It was found that even such fractional segments have distinct structural differences between themselves, and that during spreading the active spreading shifted from one segment to another. Thus, the growth of the ridge is the cumulative effect of all these local episodes. It can be seen from the profiles that even in the periods of absence of fissure eruptions, extension continues, expressed by stepped normal faults. In some segments, part of the expansion was compensated by the rise of tectonic gabbro blocks and serpentinized peridotites, i.e. rocks of layer III of the oceanic crust and lithospheric mantle.

As further deep-sea studies showed, these observations are not accidental. Zones with low spreading rates, such as the Mid-Atlantic, break up into segments, in each of which spreading proper (magmatic, constructive) alternates with phases of structural, deformational rifting, similar to continental, when the crust is stretched and thinned. In these phases, fault-limited rift valleys are formed or renewed, which, as on the continents, are symmetrical in some cases, while in others, on the contrary, they are consistent with B. Wernicke's model of deformations based on a large gently sloping fault. According to A. Carson (1992), the duration of such alternating phases reaches tens and first hundreds of thousands of years. In this case, adjacent segments of the ridge can be in different phases at the same time.

As each segment passes through a faulting extension, central rift valleys are observed in low velocity spreading zones throughout their length. Rift valleys are uncharacteristic of high-speed valleys, such as the East Pacific, and their development is clearly dominated by magmatic spreading. At the same time, the stability of the axis of fissure eruptions was noticed in them, in contrast to the zones of the Atlantic type, where lateral wandering and small “jumps” of the magmatic axis, similar to those observed under terrestrial conditions in Iceland, are not uncommon.

In the youngest spreading basins, located in a close continental frame, rapid sedimentation is possible, which prevents free fissure eruptions and the formation of a normal layer II. Before reaching the surface, dikes end up in sediments, forming sills, as has been established in the Guaymas Basin of the Gulf of California.

The volcanic zones of the mid-ocean ridges are associated with outcrops of high-temperature hydrotherms, which are especially numerous at high spreading rates. They are associated with copper-zinc pyrite ores, ferromanganese metal-bearing sediments, as well as greenstone alteration of basalts.

Formation of oceanic crust in spreading zones. Modern ideas about the mechanisms of formation of the oceanic crust are based on observations in active spreading zones in comparison with deep-sea drilling data, as well as a detailed study of ophiolites - fragments of ancient oceanic crust on the continents (see Chap. 12). The formation of layer II with a basalt upper part and a complex of parallel dolerite dikes below has already been considered above as a result of successive hydraulic wedging. The basaltic melt chambers that feed the magmatic wedges have now been delineated by multichannel seismic profiling, but only in medium- and high-velocity spreading zones. Stretching longitudinally, these foci are small in cross section, with a width of about 1 km and a height of only a few hundred meters, they are located at a depth of 1-2 km from the surface. In particular, in the East Pacific belt at 9 ° 30 "N, according to R. Detrick et al. (1937), the upper boundary of the magma chamber was traced at a depth of less than 1 km, and the newly formed oceanic crust above it is represented only by a layer II.

Stock-like bodies of massive gabbro-diabases and microgabbro intrude into such a roof in some places, which cut through a complex of parallel dikes and, in turn, can be intersected by later dike complexes.

As the newly formed crust moves away from the spreading axis, the corresponding part of the magma reservoir also moves away from the feeding system. It is no longer replenished by basalt melts of the asthenosphere, loses its connection with the main source of heat and cools under conditions favorable for crystallization differentiation (see Fig. 2.3, below). Thus, under layer II, layer III of the oceanic crust is formed - a layered complex of gabbroids, in which gradations are presented from melancocratic varieties in the upper to dunite cumulates in the lower sections. Small amounts of residual melt are sometimes squeezed out, forming small intrusions of plagiogranites comagmatic to the rest of the rock series.

Later, during the movement of the already two-layer oceanic crust from

The origin of Baikal still causes scientific controversy. Scientists traditionally determine the age of the lake at 25–35 Ma. This fact also makes Baikal a unique natural object, since most lakes, especially those of glacial origin, live on average for 10–15 thousand years, and then they are filled with silty sediments and become waterlogged. However, there is also a version about the youth of Baikal, put forward by Alexander Tatarinov, Doctor of Geological and Mineralogical Sciences in 2009, which received indirect confirmation during the second stage of the Worlds expedition to Baikal. In particular, the activity of mud volcanoes at the bottom of Lake Baikal allows scientists to assume that the modern shoreline of the lake is only 8 thousand years old, and the deep-water part is 150 thousand years old.

Some researchers explain the formation of Baikal by its location in the zone of a transform fault, others suggest the presence of a mantle plume under Baikal, and others explain the formation of the basin by passive rifting as a result of the collision of Eurasia and Hindustan. Be that as it may, the transformation of Baikal continues to this day - earthquakes constantly occur in the vicinity of the lake. There are suggestions that the subsidence of the basin is associated with the formation of vacuum chambers due to the outpouring of basalts on the surface (Quaternary period).

P.A. Kropotkin (1875) believed that the formation of the depression was associated with breaks in the earth's crust. I.D. Chersky, in turn, considered the genesis of Baikal as a trough of the earth's crust (in the Silurian). At present, the theory (hypothesis) of the "rift" has become widespread. According to this hypothesis, as a result of the compression of the earth's crust, a huge arch rise is formed, and tension, which subsequently replaces compression, causes subsidence of the upper part of the arch along the axis.

N. A. Florensov considers the Baikal basin as the central, largest and oldest link of the Baikal rift zone, which emerged and developed simultaneously with the world rift system. The "roots" of the depression, cutting through the entire earth's crust, go into the upper mantle, that is, to a depth of 50-60 km. Under the Baikal basin and, apparently, under the entire rift zone, there is an anomalous heating of the bowels, the cause of which is still unclear.

A light heated substance, rising up, lifted the earth's crust above itself, in some places cracking it to the full thickness and forming the basis of modern ridges surrounding Baikal. At the same time, the heated substance spread outward under the crust, which created horizontal tensile forces. The stretching of the crust caused the opening of ancient and the formation of new faults, the lowering of individual blocks along them and the formation of intermountain depressions - rift valleys - headed by the giant Baikal depression.

When studying the bottom sediments of Lake Baikal with the help of special piston vacuum tubes, scientists managed to select columns of bottom sediments 10-12 m long in different parts of the lake. The surface layers of bottom sediments in all basins are represented by fine-grained silty silts. But in the lower part of the cores, at a depth of 8-10 m from the bottom surface, sandy deposits were found in different places, which usually form in shallow areas of the lake or in riverbeds, in their deltas and in near-delta areas with intensive mixing of bottom sediments. However, there is nothing similar at depths of 1000-1600 m, where sandy deposits are found, at present in Baikal. Based on this, the hypothesis was born that Baikal with its great depths arose quite recently, and some researchers began to call sandy deposits under a layer of silt pre-Baikal. The rate of sedimentation in the open Baikal is currently equal to an average of 4 cm per 1000 years. Consequently, it is not difficult to calculate the time when Baikal was not yet Baikal, and in its place there were shallow reservoirs or streams - only 200-250 thousand years ago. On a geological time scale, this is quite recent, practically before the eyes of man.

The studies of paleontologists and paleolimnologists show that on Baikal, in different areas of the coast, lacustrine deposits of the Tertiary time with specific fossil lacustrine fauna - mollusks, remains of plants and other organisms are quite widespread. The age of these finds and deposits is at least 20-25 million years. Consequently, even then, on the site of modern Baikal, there was quite a lake-type reservoir with significant depths. Perhaps its outlines did not exactly match the contours of the modern lake - for example, in the southern basin it was somewhat wider. At that time, there probably was a rather deep lake in the Barguzinskaya valley and a series of lakes in the Tunka depression. The modern outlines could have formed relatively recently, perhaps in the glacial or post-glacial period, because the development of the Baikal basin, as well as the entire Baikal rift, continues - this is evidenced by numerous annual earthquakes.

And sand deposits in the thickness of bottom sediments at great depths could be formed during mudflows, turbidity flows and underwater landslides. For example, the same sandy deposits, brought by turbidity flows and underwater landslides, were found in the Pacific Ocean at a distance of several hundred kilometers from the coast of California. More thorough studies are needed, possibly with drilling of bottom sediments in the area of ​​greater depths, in order to trace the history of the development of the basin and the evolution of the animal and flora Baikal.

Rifts as global geotectonic elements are a characteristic structure of the earth's crust extension. Under the concept of rifts, narrow relief forms are also suitable - furrows (“grabens”), not yet compensated by sediments and deposits; large and wide depressions with rather mutually distant sides; dome-shaped, or stretching in the form of ridges, uplift systems complicated by an axial graben (for example, rifts in the central parts of the oceans and in East Africa). It is believed that all this is just different time stages of the formation of rift structures, which are currently found in the oceans and on the continents. Age is determined from deposits and sediments.

The first place among planetary rift systems is occupied by the World Rift System (WSR), which was formed during the Cenozoic and is currently developing, discovered in 1957, which stretches for a length of over 60 thousand km under the waters of the World Ocean, and also enters the continent by a number of its branches. . MSR are wide (up to a thousand kilometers or more) rises, rising above the bottom by 3.5 - 4 kilometers and stretching for thousands of kilometers. Active rift zones are confined to the axial parts of the ridges, consisting of a system of narrow grabens (rift gorges of the Baikal type), framed by rift mountain ranges such as the Baikal, Barguzinsky and other ridges surrounding Baikal.

Other rifts (planetary scale) include rifts confined to continents (except those mentioned above) - for example, the Rhine graben (about 600 km long) or the Baikal rift zone (more than 2.5 thousand km long). The modern rift zones of the continents have much in common with the rifts of the MSR mid-ocean ridges. Their occurrence is also associated with the processes of upwelling of deep matter, dome uplift, horizontal stretching of the earth's crust under its pressure, thinning of the crust, and uplift of the Mohorovich surface. Continental rift systems (CRS) also form extended systems branching in plan view (similar to MSR), but much less pronounced in relief, so some of their links seem to be isolated. At first glance, it is difficult to call the rift gorge, buried under a water column of 3-3.5 kilometers, an analogue of Baikal. The origin of the Baikal and oceanic rift zones is essentially the same. Most of the KSR have a Cenozoic age of formation. The Baikal rift was formed at the end of the Paleogene. In cross section, the rift zone is a system of blocks slanted at different angles, stepwise plunging towards the axial part. The interfaces are usually steeply dipping faults.

The earth's crust of continental rifts is characterized by a noticeable thinning up to 20-30 km, the rise of the Mohorovich surface and an increase in the thickness of the sedimentary layer, therefore, in the section, the earth's crust has the shape of a biconcave lens. In the study of rift structures, much has not yet been clarified and has not been studied. Is rifting a process inherent only to the Meso-Cenozoic eras? Did this process arise only in the next 100-150 million years of the life of the Earth, or should the transformation of its face be attributed to it in earlier epochs as well? These questions have not yet been clearly answered.

Rifting processes should be considered as one of the characteristic features development of the earth's crust, which took place throughout the history of her life. They are caused by horizontal stretching of the earth's crust, leading to vertical subsidence. Blocks of the earth's crust and the rise of the mantle substance to the day surface. There is a certain staging in the development of rift zones. At the first stage, a dome-shaped or linearly extended uplift is formed in the Earth's crust due to the leakage of decompacted mantle matter, then, due to extension, graben troughs form in their most elevated parts. At subsequent stages, rift zones can serve as axial parts of larger subsidences, or, in the case of change from extension to compression, they degenerate into folded uplifted structures of the geosynclinal type.

The distribution of rift zones is not strictly linear. Their individual parts (elements) are mutually displaced in the transverse direction along the transform faults. The study of modern and ancient rift zones in the ocean and on the continents will provide a clear understanding of the structure and geological history of these large geological planetary structures, as well as the oil and gas potential of many kilometers of sedimentary rocks that fill many rift depressions. Lake Baikal, as a relatively young rift zone, in its further study can provide even more extensive material for a deeper understanding of the essence of geological and magmatic processes in the area of ​​rift zones.

The Baikal rift zone is a divergent boundary located in the region of the lake and the Eastern Sayan. Its central part is located under the lake. This is where the earth's crust diverges. The Eurasian Plate is located in the west of the rift, and the Amur Plate limits it in the east, moving from the rift towards Japan at a rate of about 4 mm per year.

General information

As in other divergent zones, the earth's crust of the Baikal Rift is thinning and magma comes very close to the earth's surface. Hot springs are present both at the bottom of the lake and on the surface. However, no signs of volcanic activity have been found in the immediate vicinity of the lake's shoreline. In relatively recent times, volcanism occurred near the lake and is probably associated with the rift zone. These are the volcanic zones of the Udokan plateau, located about 400 km northeast of the upper edge of the lake, the Oka plateau with Kropotkin and Peretolchin volcanoes northwest of the southern tip, a plateau 200 km east of the rift, and the Tunkinskaya depression, located between the lakes and , which is not a submerged part of the rift. In the southwestern part of the Baikal Rift, on the territory of Mongolia, there is Lake Khubsugul.

Some researchers explain the formation of the Baikal rift by the mechanism of a transform fault, others suggest the presence of a mantle plume under Baikal, and others explain the formation of the basin by passive rifting as a result of the collision of the Eurasian plate and Hindustan. There are suggestions that the subsidence of the basin is associated with the formation of vacuum chambers due to the outpouring of basalts on the surface (Quaternary period). The Baikal Rift is active. Earthquakes constantly occur in its vicinity.

Along with the East African Rift, the Baikal Rift is another example of a divergent boundary located within the continental crust.

Application. Baikal Rift

The first geological descriptions of Lake Baikal were carried out as early as the 18th century. So, in 1772, a Russian academician, a German by birth, wrote:

“Baikal seems to be witnessing a great catastrophe; it is immeasurably deep in places, has several cliffs, like pillars, as if carved out of the depths. But in the mountains they do not find, except for unfortunate and weak earthquakes, no other destruction ... no faults, no traces of volcanoes, lavas..

Faults and volcanoes were discovered later, in the next century (their detailed study made it possible to attribute Baikal to rift structures). However, the topic of rifting became seriously interested only in the middle of the 20th century. A significant contribution to the study of the Baikal rift was made by the staff of the Institute of the Earth's Crust of the Siberian Branch of the Russian Academy of Sciences, who formed a scientific school for the study of continental rifting.

Causes of Rifting: Hypotheses

In the early 1970s, there was a wide discussion about the causes of rifting. This dispute also affected the Baikal Rift. Well-known researchers, American Peter Molnar and Frenchman Paul Tapponier, drew attention to the connection between the collision of the Asian and Indian plates with deformation in the interior of Asia. They suggested that this mechanism could lead to "passive" stretching in the Baikal rift zone. This point of view has gained great popularity abroad. Vera Alexandrovna Rogozhina and Vladimir Mikhailovich Kozhevnikov from the Institute of the Earth's Crust, using seismic data, recorded an anomalous decompression at sublithospheric depths under the Baikal Rift, in the so-called upper mantle of the Earth. Therefore, the Russian side defended the point of view about the dominant role of deep thermal processes - that is, "active" rifting. This long-term problem about the "passive" and "active" mechanism of the Baikal rift extension still remains relevant. Although recently more and more researchers come to the idea of ​​the simultaneous action of both mechanisms. The author does not impose any definite opinion on the mechanisms of formation of the Baikal rift. Instead, new, and in my subjective opinion, the most important data on tectonics, volcanism, sedimentation, and deep structure are presented. The interpretation of these data often remains ambiguous.

The structure of the Baikal rift

The Baikal Rift System is located in the interior of the continent and separates the northern stable part of the Eurasian Plate from another large stable block called the Amur Microplate. The rift system consists of a series of depressions (the largest of them is Baikal) and uplifts separating them, stretching for more than 1500 km, and also includes fields of late Cenozoic volcanism located at some distance from the depressions and their mountainous framing.

The Baikal Basin consists of two independent depressions - the South Baikal and the North Baikal, separated from each other by the Academic underwater ridge.

Scientific school for the study of continental rifting at the Institute of the Earth's Crust SB RAS (Irkutsk)

The founders of the scientific school were geologists and, as well as geophysicist Andrei Alekseevich Treskov. They laid the foundations for a systematic study of the Baikal Rift. In his autobiography (April, 1984) N.A. Florensov wrote:

"In my doctoral dissertation, it turned out to be mixed elements of coal geology, young volcanism, and most importantly, elements of the Late Mesozoic and Cenozoic tectonics of the Baikal and Transbaikalia. Earlier ..., I was looking for differences from typical African rifts, but then it turned out that there is a clear similarity between both Fortunately, my mistake turned out to be just with me, and the summary given in the dissertation and then in the monograph ... served as the starting point for extensive and many years of research on rift topics by almost our entire institute ..."

After the departure of Nikolai Alexandrovich, the baton was taken over by his closest colleague and student, an academician.

Nikolai Alexandrovich Florensov was the founder (until 1962 - the Institute of Geology of the East Siberian Branch of the USSR Academy of Sciences) and its first director. During the leadership of Nikolai Alekseevich Logachev (1976-1998), the rift theme brought the Institute wide, including international, fame. Research in this direction is still being carried out by their students and colleagues.

Age of sedimentary strata

The amount of loose sediments in the Baikal Basin is estimated at 75000 km2, which is approximately 70% of the sedimentary deposits in the basins of the entire rift system (Logachev, 2003). The South Baikal depression is considered the most ancient. In the 1970s, Nikolai Alekseevich Logachev and Nikolai Aleksandrovich Florensov suggested that its formation began in the late Eocene - early Oligocene, approximately 30-35 million years ago. Since then, this value has traditionally appeared in most publications about the Baikal Rift. IN last years Nikolai Alekseevich Logachev noted that in fact the depression could be much older.

Determination of the time of the beginning of depression formation is difficult. In order to get an answer to this question, you need to get to the rocks buried under many kilometers of sedimentary strata. Within the framework of the international project "Baikal-drilling", several wells were drilled in the Baikal sediments during the winter periods of 1996-1998. from barges frozen in ice. The longest age record was obtained when drilling sediments on the Akademichesky Ridge, since this section of the Baikal bottom is remote from all coastal sources of material drift and, therefore, is characterized by the lowest sedimentation rate. Sediments at the base of a 585 m long drilled sediment core were determined to be approximately 8.3 Ma (Horiuchi et al., 2004). This is the minimum proven age of Lake Baikal. According to the latest data, the rate of sedimentation in the last 4.5 Ma on the Akademichesky Ridge averaged about 0.04 mm per year, while earlier it averaged about 0.1 mm per year (ibid.). That is, the rate of sedimentation has decreased by more than two times! This is an unexpected result, since traditionally, according to the study of the variability of the sedimentary section of the upland depressions of the Baikal rift, the stages of "slow" Oligocene-Miocene and "fast" Pliocene-Quaternary rifting were distinguished.

In other words, the recorded change in the rate of sedimentation is directly opposite to the expected one. The only explanation for this fact, in my opinion, can be a significant uplift of the Akademichesky submarine ridge at the turn of 5-4 million years ago, which led to its isolation from terrigenous material brought mainly by the Selenga, Barguzin and Upper Angara rivers.

Modern block movement

The rate of expansion of the Baikal Basin remained until recently a subject of serious dispute. The issue was resolved thanks to the use of satellite navigation systems - GPS. Based on ten-year observations with the help of permanent and temporary GPS points, it was possible to find out that the rate of expansion of stable blocks of the Siberian Platform and the Amur microplate is 4 mm per year. In this case, all deformations are localized along the axial part of the Baikal rift.

deep structure

An important role in understanding rifting is played by studies that allow one to "see" the modern deep structure of the crust and mantle. Based on the data of seismic tomography carried out in the course of the Russian-American experiment in 1992, a velocity section of P-wave propagation was constructed (Mordvinova et al., 2003). It was found that one low-velocity anomaly is located almost under Baikal. However, the second one is located much to the south, under the territory of Mongolia, where there is no crustal extension. A reasonable question arises: what causes a decrease in the velocities of the passage of seismic waves in the mantle - an increased temperature or features of the composition of the substance? The first explanation is usually accepted.

Evolution of the Deep Thermal Regime of the Lithosphere

Partial melts from the mantle of alkaline basaltoids on their way to the surface sometimes capture fragments of surrounding rocks. The finds of such rocks, called xenoliths, are very valuable for understanding the material composition and conditions of "life" of the earth's depths. In the Baikal rift, the largest "harvest" of mantle xenoliths was collected in the eastern part of the Vitim volcanic field by Igor Viktorovich Ashchepkov and his colleagues from the Joint Institute of Geology, Geophysics and Mineralogy of the Siberian Branch of the Russian Academy of Sciences.

It turned out that mantle xenoliths from the Miocene lavas of the Vitim field indicate a large range of pressures, and they originated from great depths, from young Quaternary lavas - a smaller range. This indicates a greater thickness of the lithosphere in the Miocene under the Vitim field, in comparison with the Quaternary time. According to calculations, the thinning of the lithosphere over 13 Ma was approximately 15 km. At the same time, the boundary between the levels of formation of indicator minerals, garnets and spinels, deepened by about 8 km, which, according to experimental data, indicates an increase in temperature.

We note one more interesting feature. Despite the significant thinning of the lithosphere under the Vitim field, there was no significant stretching of the crust. According to drilling data, depressions under lavas filled with sediments do not exceed a few tens of kilometers in width, and a few hundreds of meters in depth.

Volcanism

When determining the age of the volcanic rocks of the Baikal Rift, a complex migration of volcanism was established in the Eastern Sayan and on the Udokan Range. In both areas, volcanism shifted over time along intricate trajectories with a predominant westerly trend, i.e. practically in the opposite direction from the general movement of the Eurasian lithospheric plate. This likely indicates tectonic control of magma uplift at the junction of compression and extension structures, with a general westward displacement of volcanism consistent with the existence of a relatively immobile hot magma source in the asthenosphere.

In order for a partial melt to appear in the mantle, it is necessary either to raise its temperature, or to reduce the pressure, or to saturate the mantle with volatile components. With passive rifting at a rate of 5 mm per year, as well as with such a thickness of the lithosphere and crust as in the Baikal rift, the pressure in the mantle will never decrease so much that the mantle rocks begin to melt in the absence of volatile components. However, if there are fusible areas in the mantle with water-containing minerals or carbonates, then such areas, even with slight temperature and pressure drops, will pass into the melt.

Characteristically, the distribution of volcanic fields tends neither to rift depressions nor to gravitational minima, areas of potential heat increase. A particularly indicative example is the Dariganga volcanic plateau in Mongolia. Apparently, this indicates that the melting of the mantle of the Baikal Rift and adjacent territories is controlled, first of all, by its composition.

To study the composition of the melting mantle, the isotope ratios of the elements are studied. The ratio of neodymium and strontium isotopes measured in the lavas of the southwestern part of the Baikal Rift, in comparison with the compositions of the lavas of the Khangai Ridge, showed that the mantle melting region can be divided into three parts (arbitrarily designated as components A, B, and C). Component A refers to the region of the sublithospheric mantle (asthenosphere), and the other two components characterize the inhomogeneous lithospheric mantle. Moreover, component B can refer to the deeper parts of the garnet-containing mantle, and component C, to the spinel-containing mantle or the region of the crust-mantle transition.

There are two extreme models of lithospheric extension in the inland regions, called the "active" and "passive" rifting models. The driving force behind "active" rifting is the heat source of the ascending mantle flow, commonly referred to as the plume. It is assumed that the region of origin of such plumes can be located at the section of the upper and lower mantle at a depth of 650 km or even at the boundary with the core at a depth of 2700 km.

The main characteristics of "active" rifting are the formation of tectonic basins against the background of a large regional uplift, an increased heat flow, and widespread volcanism. The latter must precede both the formation of a regional uplift and the formation of a depression. The predominant composition of the volcanic rocks of an "active" rift should manifest itself over a large area and not depend on the composition and age of the lithosphere.

In the model of "passive" rifting, the main cause of extension is considered to be tectonic stresses that occur at the boundaries of lithospheric plates at a considerable distance from the extension area. The fixed uplift of the sublithospheric mantle passively follows the thinning of the lithosphere. A characteristic of "passive" rifts is the confinement of all rift structures to the ancient boundaries between lithospheric blocks different ages and weakly manifested volcanism. At the same time, extension precedes volcanism, and volcanic rocks reflect the heterogeneous composition of the lithosphere.

Correlation of tectonic events

Only crustal stresses from the zone of the Indo-Asian collision or local sources of heat in the mantle could not lead to the formation of the Baikal rift. In recent years, the idea of ​​the important role of the interaction of lithospheric plates on the eastern margin of Eurasia has also been discussed.

It is noteworthy that episodes of compression and extension in the collision zones of the Indo-Asian and Pacific-Asian plates are shifted relative to each other in time. If compression acted on the southern margin of Central Asia, then at that time there was an extension regime on its eastern margin. And, conversely, the significant compression that occurred on the eastern margin, the southern margin experienced an episode of relaxation.

Such dynamics of compression and extension could "swing" the inner parts of Central Asia, lead to the displacement of blocks, which, given their geometry, formed zones of compression and extension at the boundaries of these blocks. With such a mechanism, it should be expected that the impulses of the main tectonic events in Central Asia (for example, the rotation impulses of the Amur microplate) will coincide in time with the change in the tectonic regime at the boundaries of the lithospheric plates. Unfortunately, the dating of such pulses is still a difficult task. For the Baikal rift, uplift periods can be estimated from data on the position of dated lavas in the relief. In total, 4 such episodes were identified: 21-19, 16-15, 5-4 and about 0.8 million years ago. It is interesting that the change in the rate of sedimentation on the underwater Akademichesky Ridge, which occurred 5-4 million years ago, coincided with one of such episodes of uplift. As noted earlier, this event may mark the beginning of the stage of "rapid" rifting. At that time, there was an expansion regime in the front of the Indo-Asian collision, and compression on the eastern margin of Central Asia began a little earlier than this episode. Thus, the stage of "rapid" rifting cannot be genetically related to distant tectonic events in the front of the Indo-Asian collision. It is associated either with tectonic events on the eastern border of Asia, or with thermal and/or chemical effects on the lithosphere due to local mantle heat sources.

Conclusion

So what is the Baikal rift after all - “active” or “passive”?

Crustal deformations and extensions are mainly controlled by distant tectonic events occurring at lithospheric plate boundaries. Heating, thinning and melting of the lithosphere are carried out due to deep sources of heat, or due to the existence of light-melting regions in the mantle. This means that the Baikal rift bears the features of both "active" and "passive" rifting. Trying to consider the development of the Baikal rift solely from the standpoint of studying crustal deformations or the evolution of volcanism, or deep geophysics, we find ourselves in the position of blind wise men who study the elephant by touch in a well-known parable. Only the integration of various research areas will allow us to answer which of the mechanisms of rifting prevailed, whether their ratio changed over time, whether the processes of crustal extension and magma formation are related, or are these two independent processes. The need to combine their efforts today is recognized by almost all researchers, which means that someday, when starting an article about the Baikal Rift, it will be possible to say “we know how and why it was formed.”